1. Field of the Invention
The present invention relates to a solid state imaging device which has an on-chip color filter and a method for producing the same.
2. Description of the Prior Art
In recent years, single-plate color solid state imaging devices has been remarkably developed along with the advancement of color picture imaging. Typical examples of such devices include, for example, a solid state imaging device for digital still cameras, mainly including CCD (Charge Coupled Device)-type devices, and a solid state imaging device for mobile camera phones, mainly including CMOS (Complementary Metal Oxide Semiconductor)-type devices. Therefore, demands for downsizing and increase in the number of pixels on the solid state imaging device having an on-chip color filter have been increasing.
However, such demands on the solid state imaging device result in a reduction of the light receiving area (the area of a photodetection sensor) of a photoelectric conversion element, which constitutes a cause of the decrease in the photoelectric conversion characteristic (photosensitivity) which is a primary characteristic of the solid state imaging device. Further, a color filter layer formed on the photoelectric conversion element is also downsized. Thus, conventional techniques cannot follow such severe demands as to reduction in film thickness and miniaturization and dimensional accuracy of a color filter layer. Accordingly, the characteristics of the color imaging device are deteriorated due to the color filter layer, so that color mixture, uneven color, nonuniform tone of lines, black defects, etc., occur.
For example, the typical size of optics incorporated in the most popular digital still cameras is shifting from ⅓ inch to ¼ inch, while a digital still camera with ⅙-inch or smaller optics has been developed for future products. The typical number of pixels is shifting from 3 Mega (3,000,000) pixels to 5 Mega (5,000,000) pixels. Further, a digital still camera with 6 Mega (6,000,000) pixels has been developed for future products.
In such a solid state imaging device with a decreased light receiving area and an increased number of pixels, new techniques need to be established for preventing deterioration in photosensitivity, which is one of the principal characteristics of the solid state imaging device, and preventing color mixture between adjacent pixels, uneven color, nonuniform tone of lines, black defects, etc.
That is, if only the number of pixels is increased without decreasing the pixel size, the chip size is increased, and the size of the solid state imaging device increases. Therefore, the size reduction and the increase in the number of pixels cannot be simultaneously achieved without a decrease in pixel size. In general, when the pixel size is decreased, the size of the photoelectric conversion element, typified by a photodiode, is accordingly decreased. Thus, a decrease in photosensitivity cannot be avoided. In view of such, various countermeasures have been devised for improving the photosensitivity. Especially for a microlens formed on a pixel, various ideas have been proposed as to both structure and production method.
The reduction of the pixel size (miniaturization) causes not only deterioration in photosensitivity but also deteriorations in various color characteristics due to a color filter layer. That is, in general, the dimensional accuracy of the color filter layer deteriorates along with a decrease in pixel size, and therefore, the characteristics of the solid state imaging device are deteriorated so that color mixture between adjacent pixels, nonuniform tone of lines, variation in photosensitivity, uneven color, etc., occur.
Conventionally, the color filter layer is formed of a so-called “dye-type” material. The dye-type material is a photosensitive material prepared by mixing a water-soluble polymer, such as gelatin, casein, polyvinyl alcohol, or the like, and a photosensitive cross-linking agent, such as a chromate, a dichromate, or the like. The dye-type material is dyed with an acid dye, or the like. To form a color filter layer of a dye-type material, in the first step, application, exposition and development are performed using a photosensitive material to form a pattern. Then, the resultant structure is dyed with an acid dye, or the like, and then, an anti-dyeing layer is formed of an acrylic film, or the like, whereby a color filter layer is completed. Alternatively, instead of providing the anti-dyeing layer, an anti-dyeing treatment may be carried out using tannic acid, or the like, on a dyed photosensitive material film before the color filter layer is completed.
Presently, color filters formed of a so-called “pigment-dispersed” material are more popular than the dye-type color filters. The pigment-dispersed material is a material prepared by dispersing a pigment in a binder resin and adding a cross-linking agent thereto for providing photosensitivity. The pigment-dispersed material can be handled, i.e., applied, exposed, and developed, as is a commonly-used resist, or the like, to form a pattern thereof, whereby a color filter layer is completed. The color filter layer of such a pigment-dispersed material can be formed into a thinner layer as compared with a conventional material and is excellent in heat resistance, lightfastness and chemical resistance. Further, with such a material, the production process can be simplified.
Comparing a dye-type color filter layer and a pigment-dispersed color filter layer, the dye-type color filter layer has the following characteristics.
An advantage of the dye-type color filter layer is a variety of dyes available, and hence, a high selection flexibility toward the spectral characteristics. Further, since the dye itself is not in the form of particles, the frequency of occurrences of black defects is low when such a dye-type material is used for a color filter layer.
A disadvantage of the dye-type color filter layer is the necessity for a thicker film for the purpose of achieving desired spectral characteristics. Because of this, the definition is deteriorated, and this is disadvantageous in finer pattern formation. As for durability (heat resistance, lightfastness, chemical resistance, etc.), the dye-type material is inferior to the pigment-dispersed material. Further, dyeing and anti-dyeing steps are necessary. Therefore, a production process becomes longer, and there are many factors to vary the spectral characteristics, such as concentration, temperature, pH, time, etc.
At least as a solution to the disadvantage of the dye-type color filter, the pigment-dispersed color filter layer has been mainly used as of now. However, the colorant which determines the spectral characteristics is in the form of particles, and therefore, the number of black defects is nonnegligibly large as a result of the decrease in pixel size, as to which the pigment-dispersed color filter is inferior to the dye-type color filter.
In view of such, for the purpose of ameliorating a black defect which would occurs due to a pigment-dispersed color filter layer, a so-called “dye-contained type” material has been proposed and practically used in some devices. The dye-contained type material is a material prepared by dispersing a dye in a binder resin and adding a cross-linking agent thereto for providing photosensitivity.
When a dye-contained type color filter layer is used, a black defect can be ameliorated, and in addition, the production process does not require an anti-dyeing step. Thus, the dye-contained type color filter layer can be formed through substantially the same process as that of the pigment-dispersed color filter layer. However, the dye-contained type color filter layer is inferior to the pigment-dispersed color filter layer in readiness to reduce the film thickness and durability (lightfastness, heat resistance, chemical resistance, etc.).
As described above, the significance of the on-chip color filter layer in the solid state imaging devices has been increasing, and establishment of a technique which overcomes various disadvantages, such as color mixture, nonuniform tone of lines, uneven color, black defects, etc., has been demanded. In the conventional solid state imaging devices, dye-type, pigment-dispersed type, and dye-contained type color filter layers are selectively used according to their uses. However, none of these color filter layers overcomes all the disadvantages.
Under such circumstances, the researchers have been studying for eliminating the disadvantages of the above materials. For example, a method for forming a dye-type color filter layer (first color filter layer) and a pigment-dispersed color filter layer (second color filter layer) on the dye-type color filter layer has been recommended. A conventional solid state imaging device 10 having such a layered color filter has been described below.
FIG. 6 is a cross-sectional view of the conventional solid state imaging device 10, which schematically shows a structure of photoelectric conversion elements and their peripheral elements.
The conventional solid state imaging device 10 is formed using, for example, an N-type semiconductor substrate 11. On the N-type semiconductor substrate 11 is a P-type well layer 12. On the P-type well layer 12 are a plurality of photoelectric conversion elements 13, which constitute an N-type semiconductor layer.
The conventional solid state imaging device 10 includes a gate dielectric film 14, which covers the P-type well layer 12 and the photoelectric conversion elements 13, and transfer electrodes 15 formed on the gate dielectric film 14 for transferring signals. The transfer electrodes 15 are provided above the interval areas between the photoelectric conversion elements 13.
The conventional solid state imaging device 10 further includes an interlayer dielectric film 16, which covers the transfer electrodes 15, and a light shielding film 17 which covers the interlayer dielectric film 16 and prevents light from reaching the transfer electrodes 15.
The conventional solid state imaging device 10 further includes a surface protection film 18, which covers the gate dielectric film 14 and the light shielding film 17, and a first transparent flattening film 21a which compensates for the convexities/concavities generated by the transfer electrodes 15.
On the first transparent flattening film 21a is a first color filter layer 19. The first color filter layer 19 has predetermined colors for respective pixels. For example, the first color filter layer 19 includes first green filter segments 19G, first blue filter segments 19B, and first red filter segments 19R.
On the first color filter layer 19 is a second color filter layer 20. The second color filter layer 20 also has predetermined colors for respective pixels. For example, the second color filter layer 20 includes second green filter segments 20G, second blue filter segments 20B, and second red filter segments 20R. In each pixel, the first color filter layer 19 and the second color filter layer 20 have filter segments of the same color.
On the second color filter layer 20 is a second transparent flattening film 21b which provides a flat surface over the structure. On the second transparent flattening film 21b are on-chip microlenses 22 provided for improving the light-collection efficiency. The microlenses 22 correspond to the pixels on a one-to-one basis.
In the solid state imaging device 10, the first color filter layer 19 is a dye-type color filter layer, and the second color filter layer 20 is a pigment-dispersed color filter layer. With such a structure, the disadvantages of the dye-type color filter layer in heat resistance, lightfastness and chemical resistance are eliminated by the pigment-dispersed color filter layer superposed thereon.
Such a layered color filter is disclosed in, for example, Japanese Laid-Open Patent Publication No. 5-119211.
However, the above-described conventional technique has other disadvantages as described below.
In the production process of the solid state imaging device 10, the first color filter layer 19, which is a dye-type color filter layer, is formed through application of a dye-type material, pattern formation by exposure and development, and dyeing and anti-dyeing steps. The second color filter layer 20, which is a pigment-dispersed color filter layer, is formed through application of a pigment-dispersed material on the first color filter layer 19 and pattern formation by exposure and development.
That is, the steps of application, exposure and development are performed twice (i.e., performed separately on two color filter layers). Herein, different developer solutions are used for the first color filter layer 19 and the second color filter layer 20. Specifically, the developer solution for the dye-type color filter layer is pure water, while the developer solution for the pigment-dispersed color filter layer is an organic alkaline solution.
Since the exposure step is performed twice, the possibility of mask misalignment is increased. Accordingly, it is difficult to perform pattern formation with a precision desired for the color filter. That is, misalignment is likely to occur between the two color filter layers. As a result, the problems of uneven color, nonuniform tone of lines, color mixture between adjacent pixels, etc., are not sufficiently overcome.
In the solid state imaging device 10 shown in FIG. 6, color boundaries are misaligned between the first color filter layer 19 and the second color filter layer 20. For example, the boundary between the first green filter segment 19G and the first blue filter segment 19B is not coincident with the boundary between the second green filter segments 20G and the second blue filter segments 20B.
Thus, for example, light 30 diagonally incident on the solid state imaging device 10, which is indicated by an arrow, passes through filter segments of different colors (the second green filter segments 20G and the first blue filter segment 19B) before reaching the photoelectric conversion element 13.
Further, conventionally, to compensate for insufficient adhesion between a surface protection film and a color filter layer, a transparent flattening film is provided between the surface protection film and the color filter layer. Accordingly, the total thickness of the layers formed on the photoelectric conversion element increases (the distance between the photoelectric conversion element and the microlens increases). Thus, color mixture due to diagonal incident light increases, and the incident angle dependence deteriorates.